null

function getVowels(str) {
  const m = str.match(/[aeiou]/gi);
  if (m === null) {
    return 0;
  }
  return m.length;
}

console.log(getVowels("sky"));
// Expected output: 0

//  foo   -        :
> foo
'ReferenceError: foo is not defined'

//  foo ,      ,  :
> var foo = null; foo
'null'

var element;
//   element     : undefined

element = document.getElementById("not-exists");
//      ,  getElementById  null
//  element    null,   

typeof null; // object ( "null"    )
typeof undefined; // undefined
null === undefined; // false
null == undefined; // true